It is often necessary or desirable to secure building members or machinery parts to a concrete wall after the wall is hardened, and in such cases it is customary to drill blind holes in the wall to receive anchor bolts or similar fastening means, of which many types are known in the prior art that are adequate for ordinary purposes.
In the building of nuclear power plants, however, the requirements have much closer tolerances, for example, as to pullout resistance, permissible longitudinal shifting in the hole when expanded, precautions to be taken to insure that the concrete will not be overstressed or cracked by expansion of the anchor, etc. To meet these requirements the present invention provides a type of anchor which has an annular collar which expands into a complimentary shaped undercut hole, the shape of the anchor being improved over prior art configurations.
The prior art shows several anchors which have radially expandable enlargements at their inner ends which are adapted to be set in an annular undercut located at the inner end of a blind hole, for instance as shown in Italian Patent No. 519,037 and in British Patent No. 618,489. In U.S. Pat. No. 2,707,897 to Beeson there is shown an anchor which is rotated in a hole to form its own undercut groove in the blind end thereof. Other patents showing holes preformed with undercuts at their inner ends include U.S. Pat. No. 519,172 to Calkins, U.S. Pat. No. 1,025,275 to Kennedy and U.S. Pat. No. 3,403,594 to Newell.
Examples of patents showing expandable anchors having annular shoulders which are expanded to dig into the walls of a hole and distort the walls by compressing the wall material are included in French Patent No. 2,307,166, and in the following U.S. prior art including U.S. Pat. No. 734,326 to Hicks, U.S. Pat. No. 1,179,472 to Tarleton, U.S. Pat. No. 2,561,433 to Uhle, U.S. Pat. No. 3,217,583 to Fulop, and U.S. Pat. No. 3,620,120 to Warner. Such compression of the wall material tends to cause overstressing in a concrete wall, and raises a risk of cracking the concrete which can not be permitted in a nuclear installation.
When an undercut blind hole is to be provided in a cured concrete wall, or the like, it is formed in two successive cutting operations. First, a cylindrical blind-end hole is drilled with a suitable masonry drill, for instance having carbide and/or diamond inserts, this hole being slightly larger in diameter than the body of the anchor to be set therein, and being at least as long as the portion of the anchor which is to occupy the hole. Second, an undercutting tool is inserted in the blind hole and is rotated therein to form an undercut groove at the desired axial position within the hole. Suitable undercutting tools are shown in the prior art, for instance in FIG. 8 of the aforementioned British Pat. No. 618,489, or in U.S. Pat. No. 2,365,549 to Haynes or U.S. Pat. No. 2,457,269 to Pfrehm.
In the case of each of the prior art anchors mentioned three paragraphs above as showing annular enlargements which are expandable into a previously undercut groove in the hole, the undercut groove and the enlargement were placed at the extreme inner end of the anchor and the hole. When an anchor having an annular locking collar has been expanded into locked position there are two shoulders to be considered in determining whether or not the anchor can be axially shifted in the hole, namely the forward facing shoulder of the collar which is an abutment resisting shifting of the anchor in the pull-out direction, and the rearward facing shoulder of the collar which is an abutment resisting shifting of the anchor further into the hole. The problem involved in successfully seating the anchor without leaving it axially shiftable results from the fact that two different tools are used to form the undercut shoulder abutments of the hole. It is a masonry drill which forms the blind end of the hole, but it is an undercut tool which forms the abutment for the forward facing shoulder. As a result, the separation between these inner and outer abutment shoulders as provided by the innermost wall of the hole and the undercut groove shoulder can be a variable, dependent in magnitude partly upon the skill of and partly upon the care taken by the workman in forming the hole.